The present invention relates to bi-directional signalling and communication and, more particularly, to duplexing.
In the copending application of Raymond L. Barrett, Jr., Ser. No. 354,156 now U.S. Pat. No. 4,471,345 filed Mar. 5, 1982, for RANDOMIZED TAG TO PORTAL COMMUNICATION SYSTEM, and assigned to the same assignee as the present application, there is described a system whereby the whereabouts of personnel in a hospital or other facility is determined and stored through the interaction of identification tags and portal interrogating stations. Each individual to be monitored is provided with a unique identification tag, and each portal is provided with an encircling magnetic loop and associated circuitry for establishing communication with any tags that come within range. As described in said application, the portals are provided with single loops and infer that each time a tag comes within range it passes through. Although physical constraints can be applied to doorways and passageways so as to minimize detection of a tag unless it is being carried through, the need for such constraint is undesirable and individuals do, on occasion, change their mind. Preferably, the system should be direction sensitive so as to recognize on which side of the portal a tag is located regardless of prior history.
In the copending application of Henry F. Pfister, Ser. No. 413,612, filed Sept. 1, 1982, now U.S. Pat. No. 4,489,313 for SIGNAL DIRECTION DETERMINING SYSTEM AND DIRECTIONAL LOOP ANTENNA ARRAY THEREFOR, and assigned to the same assignee as the present application, there is described a system for using the signals from a pair of spaced open loops to provide directional information. The signals from the open loops are vectorially added and subtracted in a sum and difference circuit and the phase angle between the sum and difference signals is ascertained in a phase detector circuit that feeds an indicator. The so determined phase angle is either greater or less than 90.degree. depending upon the relative magnitudes of the loop signals, which, in turn, is indicative of the direction to the signal source.
The present invention addresses itself to the problem of incorporating the direction determining system of said Pfister application in the system of said Barrett application. For this purpose it becomes necessary to provide a system whereby an interrogating signal can be communicated to and radiated by a pair of open loop antennas while avoiding loading of the signal by the direction determining and signal receiving apparatus. At the same time, the significantly weaker responses from identification tags must be operated on to provide the sum and difference components and passed efficiently to the direction determining and receiving components.